1. Field of the Invention
The present invention relates to noise reduction in reluctance machines.
2. Description of Related Art
In general, a reluctance machine is an electrical machine in which torque is produced by the tendency of its movable part to move into a position where the reluctance of a magnetic circuit is minimized, i.e. where the inductance of the exciting winding is maximized. In one type of reluctance machine, the energization of the phase windings occurs at a controlled frequency. This type is generally referred to as a synchronous reluctance machine, and it may be operated as a motor or a generator. In a second type of reluctance machine, circuitry is provided for detecting the angular position of the rotor and energizing the phase windings as a function of the rotor position. This second type of reluctance machine is generally known as a switched reluctance machine and it may also be a motor or a generator. The characteristics of such switched reluctance machines are well known and are described in, for example, "The characteristics, design and application of switched reluctance motors and drives" by Stephenson and Blake, PCIM '93, Nurnberg, Jun. 21-24, 1993, which is incorporated herein by reference. The present invention is generally applicable to reluctance machines, including switched reluctance machines operating as motors or generators.
FIG. 1 shows the principal components of a typical switched reluctance drive system. The input DC power supply 11 can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11, which can be fixed or variable in magnitude, is switched across the phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive. A rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. The output of the rotor position detector 15 may also be used to generate a speed feedback signal.
The rotor position detector 15 may take many forms; for example it may take the form of hardware, as shown schematically in FIG. 1, or of a software algorithm which calculates the position from other monitored parameters of the drive system, as described in EP-A-0573198 (Ray), which is incorporated herein by reference. In some systems, the rotor position detector 15 can comprise a rotor position transducer that provides output signals that change state each time the rotor rotates to a position where a different switching arrangement of the devices in the power converter 13 is required.
The energization of the phase windings in a switched reluctance machine depends heavily on accurate detection of the angular position of the rotor. The importance of accurate signals from the rotor position detector 15 may be explained by reference to FIGS. 2 and 3, which illustrate the switching of a reluctance machine operating as a motor.
FIG. 2 generally shows a rotor pole 20 approaching a stator pole 21 according to arrow 22. As illustrated in FIG. 2, a portion 23 of a complete phase winding 16 is wound around the stator pole 21. As discussed above, when the portion of the phase winding 16 around stator pole 21 is energized, a force will be exerted on the rotor, tending to pull rotor pole 20 into alignment with stator pole 21. The pole faces of both rotor and stator poles are defined by arcs having their centers on the rotational axis of the rotor. The angular extent of these arcs is a matter of choice by the designer. The traditional methods for choosing the angular extents of the arcs are discussed in "Variable-speed switched reluctance motors" by Lawrenson et al, IEE Proc., Vol. 127, Pt. B, No. 4, July 1980, pp. 253-265, which is incorporated herein by reference. It will be recognized by those skilled in the art that, as a consequence of the arcuate pole faces, the distance between the overlapping pole faces of the rotor and stator, when measured along a radius from the rotational axis, is constant.
FIG. 3 generally shows typical switching circuitry in the power converter 13 that controls the energization of the phase winding 16, including the portion 23 around stator pole 21. When switches 31 and 32 are closed, the phase winding is coupled to the source of DC power and is energized. Many other configurations of switching circuitry are known in the art; some of these are discussed in the Stephenson & Blake paper cited above.
In general, the phase winding is energized to effect the rotation of the rotor as follows. At a first angular position of the rotor (called the "turn-on angle", .theta..sub.ON) the controller 14 provides switching signals to turn on both switching devices 31 and 32. When the switching devices 31 and 32 are on, the phase winding is coupled to the DC bus, causing an increasing magnetic flux to be established in the machine. The magnetic flux produces a magnetic field in the air gap which acts on the rotor poles to produce the motoring torque. The magnetic flux in the machine is supported by the magneto-motive force (mmf) which is provided by a current flowing from the DC supply through the switches 31 and 32 and the phase winding 23. In some controllers, current feedback is employed and the magnitude of the phase current is controlled by chopping the current by rapidly switching one or both of switching devices 31 and/or 32 on and off. FIG. 4(a) shows a typical current waveform in the chopping mode of operation, where the current is chopped between two fixed levels. In motoring operation, the turn-on angle .theta..sub.ON is often chosen to be the rotor position where the center-line of an inter-polar space on the rotor is aligned with the centerline of a stator pole, but may be some other angle.
In many systems, the phase winding remains connected to the DC bus (or connected intermittently if chopping is employed) until the rotor rotates such that it reaches what is referred to as the "freewheeling angle", .theta..sub.FW. When the rotor reaches an angular position corresponding to the freewheeling angle (e.g., the position shown in FIG. 2) one of the switches, for example 31, is turned off. Consequently, the current flowing through the phase winding will continue to flow, but will now flow through only one of the switches (in this example 32) and through only one of the diodes 33/34 (in this example 34). During the freewheeling period, the voltage drop across the phase winding is small, and the flux remains substantially constant. The circuit remains in this freewheeling condition until the rotor rotates to an angular position known as the "turn-off angle", .theta..sub.OFF (e.g. when the centerline of the rotor pole is aligned with that of the stator pole). When the rotor reaches the turn-off angle, both switches 31 and 32 are turned off and the current in phase winding 23 begins to flow through diodes 33 and 34. The diodes 33 and 34 then apply the DC voltage from the DC bus in the opposite sense, causing the magnetic flux in the machine (and therefore the phase current) to decrease.
As the speed of the machine rises, there is less time for the current to rise to the chopping level, and the drive is normally run in a "single-pulse" mode of operation. In this mode, the turn-on, freewheel and turn-off angles are chosen as a function of, for example, speed and load torque. Some systems do not use an angular period of freewheeling, i.e. switches 31 and 32 are switched on and off simultaneously. FIG. 4(b) shows a typical such single-pulse current waveform where the freewheel angle is zero.
It is well known that the values of turn-on, freewheel and turn-off angles can be predetermined and stored in some suitable format for retrieval by the control system as required, or can be calculated or deduced in real time.
When the phase winding of a switched reluctance machine is energized in the manner described above, the magnetic field set up by the flux in the magnetic circuit gives rise to the circumferential forces which, as described, act to pull the rotor poles into line with the stator poles. In addition, however, the field also gives rise to radial forces which are generally an order of magnitude greater than the circumferential forces, and which principally act to distort the stator structure radially, by pulling the stator radially towards the rotor. This is often termed "ovalizing". Such forces are described in detail, for example, in "Influence of stator geometry upon vibratory behavior and electromagnetic performances of switched reluctance motors", by Picod, Besbes, Camus, & Gabsi, IEE EMD97, Eighth International Conference On Electrical Machines and Drives, Sep. 1-3, 1997, Robinson College, Cambridge, UK, pp. 69-73, which is incorporated herein by reference. The effect of these ovalizing forces on the vibrational behavior of the stator structure is analyzed and discussed in EPA 0763883, which is incorporated herein by reference. It is well known in the art that these vibrational forces can give rise to acoustic noise being emitted from the structure of the machine or from other components to which the machine is attached.
Many methods of modifying these forces, and hence reducing the acoustic noise, have been suggested in the past. The methods have generally approached the problem by modifying, in some way, the excitation supplied to the phase winding. For example, "Analysis and reduction of vibration and acoustic noise in the switched reluctance drive", Wu & Pollock, IEEE IAS Conference, Toronto, Oct. 2-8 1993, pp. 106-113, which is incorporated herein by reference, discusses a proposal for using a predetermined amount of freewheeling to reduce noise.